1. Field of the Invention
This invention relates generally to a system for making endless belts reinforced with a spiraled tensile cord layer, more particularly a system for making toothed belts of practically any length on the same apparatus and tooling and specifically endless belts with no cord exposed to the outside surfaces of the belt.
2. Description of the Prior Art
Endless belts are typically made of elastomers having tension members embedded in the circumferential direction. The elastomer may be vulcanized rubber, thermoplastic elastomer, or castable elastomers. There are several methods typically used for manufacturing endless belts of thermoplastic elastomers and of rubber.
A first method uses a cylindrical mandrel of a defined diameter to produce a specific endless belt length corresponding to that diameter. Such a mandrel may include the desired surface patterns or profile on its surface to produce for example a toothed belt profile. An example as applied to thermoplastic belts is disclosed in GB 886,754 to Hutzenlaub. These mandrels are expensive and this method can only produce one specific belt length on a given mandrel. The longer the belt, the bigger and more expensive the mandrel. Belt length is therefore limited for practical purposes. Note that belt “length” for an endless belt refers to the circumference of the belt. This method is also used for making endless rubber belts, as disclosed for example in U.S. Pat. No. 3,078,206, to Skura. This kind of method is also common for castable elastomer belts.
A second method particularly adapted to making endless belts from open-ended belts is to produce a continuous length of reinforced belt material which is subsequently cut to the desired length and the two ends joined together to make an endless belt. Various splicing or joining methods are in use, but the splice is always weaker than if the tensile cord was continuous and helically wound. Examples of the continuous method for making open-ended thermoplastic belts are disclosed in U.S. Pat. Nos. 3,880,558 and 4,251,306 to Breher, et al., in which a rotatable molding wheel is supplied with a molding band (usually of flexible steel) wrapping around about half of the circumference of the molding wheel to form a rotating molding chamber into which cords are fed along with extruded belt material. Likewise, a continuous length of rubber belt can be made by advancing the belt elements between a heated grooved cylindrical mold and a pressure band. In a secondary process, the ends of the open-ended belt having the desired belt length are joined together. Such a splice causes dimensional irregularities (or pitch error) and provides a weak point in the belt, typically reducing the load capability and lifetime of the belt by about 50% vs. a non-spliced endless belt.
A third method uses two cylindrical mandrels which can be moved relative to each other to adjust for the desired belt length. Typically, the method includes helically winding the tension members around the two mandrels, and then extruding or casting and/or curing an elastomeric material to provide the elastic matrix, embedding the tension member and forming the profile of the belt. Typically, one of the mandrels is a molding wheel with a pressure band forming a molding chamber into which belt material is injected or extruded as described in the second method above. This method has some disadvantages. The equipment is expensive and space consuming, especially for longer belt lengths, and the operating efficiency and output rate is less than desirable. The belt length is limited on the low end by the minimum distance between the two mandrels. Belt length is limited on the high end by the maximum distance technically feasible to control the accuracy of the center distance of the belt. Center distance variation can also be a problem as a result of the gradually increasing total tension between the two mandrels as the cord is wound on under tension. Mold flights are typically used to support the tension members, resulting in cord exposure in the finished thermoplastic belt. Rubber belts according to this method could be cured in a series of steps in which the belt materials are progressively advanced between flat molding plates.
In a fourth method, an open-ended strip of belt is helically wrapped around two mandrels spaced to achieve a desired belt length, and the edge seams are fused or glued together to form an endless belt of desired belt width. This method permits belts of different length by changing the distance between the two mandrels. An example is disclosed in U.S. Pat. No. 4,058,424 to Breher. This method also has some disadvantages. The equipment is expensive and space consuming, and the operating efficiency and output rate is less than desirable. In addition, depending on the width of the strips, more cords are cut and exposed on the belts edges, reducing the effective strength of the belt, slight differences in the tension of the strips causes pitch variation and sideways tracking of the belt, resulting in reduced belt life and noise. The belt length is limited on the low end by the minimum distance between the two mandrels. Belt length is limited on the high end by the maximum distance technically feasible to control the accuracy of the center distance of the belt. Further reliable joining of the strips is difficult and represents a potential failure risk, causing the belt to disintegrate during higher load conditions, particularly by cutting the ends of the strip on the belt edge against a pulley flange and then peeling or unraveling the belt.
Mention is made of the applicants' co-pending U.S. application Ser. No. 13/715,977, titled “Method of Making Open-Ended Thermoplastic Belting,” filed on the same day, claiming the benefit of provisional application 61/570,815 filed on Dec. 14, 2011, the entire contents of which are hereby incorporated herein by reference.
What is needed is a more efficient, accurate method of making endless reinforced belts without need for a splice, no exposed cord, and using one tool to make different belt lengths.